EMYDID&. 285 
a right angle with the scapula. Coracoids not expanded at the distal ends. Humerus with the 
radial and ulnar processes diverging. Digits usually elongated and the median ones furnisht 
with three phalanges; only in T errapene are the median anterior digits reduced to 2 phalanges. 
Feet fitted primarily for walking, but nearly always provided with a web between the toes. 
Skull without temporal roof, except the zygomatic and postorbital arches, the former 
occasionally wanting. Stapedial passage open. Neck completely retractile within the shell. 
In the present work the writer, following by conviction the example of a large number of 
students of herpetology, the names of ns es are mentioned by Louis Agassiz in his Contri- 
butions to the Natural History of the United States (vol. 1, p. 356), has separated the true land- 
tortoises, the Testudinidz, from the more or less aquatic forms that are to be known as the 
Emydidz. For this course there appears to be found in the structures of the two groups 
abundant justification. Furthermore, the two families have been clearly separated ever 
since the time of the Lower Eocene. 
Even as thus restricted the Emydidz contain more genera and species than any other family 
of Testudines. According to Mr. G. A. Boulenger’s Catalogue of Chelonians, the Emy- 
dide, excluding the land-tortoises, embrace seventeen genera and sixty-four species; and 
these numbers are to be increast, rather than diminisht. Probably no other family displays 
a greater range of habits. Some of the species are almost w holly aquatic, as those of the 
genera Chrysemys and Graptemys; others are semiaquatic, as the members of Emys and 
Clemmys; while still others, as Terrapene, are nearly as terrestrial as the Testudinidae them- 
selves. Of the aquatic forms a few, as the diamond-back terrapins (Malaclemys) affect the 
salt waters of the sea-coasts; while the greater number inhabit fresh-water lakes and streams. 
Most of the species are carnivorous in their diet, but a few, as the box-tortoises (J errapene) 
subsist on vegetation. 
As regards their present-day distribution, the Emydide are found occupying portions of 
all the continents and the larger islands of the warmer portions of the globe, except Australia 
and the neighboring islands (fg. 13, page 33). It is evident that by some ancient connection 
of Australia with other lands, probably with Asia, the Pleurodires were enabled to occupy 
that southern continent, as well as New Guinea; but this connection was interrupted before 
the Emydidz and the Testudinid had passed over that bridge. 
Altho the Emydidz have secured a foothold in the African continent, their distribution is 
limited to the northwestern and northern coasts. Only 2 species are known from Africa— 
Emys orbicularts and Clemmys leprosa. The first is widely spread over Europe and south- 
western Asia and is found in Algeria. It is within the range of possibility that it has 
been introduced into Algeria through human agency. Clemmys leprosa is found along the 
southern Mediterranean coast, from Tunis to the Atlantic, thence southward to Senegal. 
It has evidently entered Africa at a late geological period and has prest southward along 
the Atlantic coast until it has reacht the border of the Ethiopian region. To turtles, as to 
other groups of animals, the Sahara desert has for a long period stood as a barrier to their 
southern migration. 
Itisa ae difficult, perhaps impossible, of explanation at present, that while the Emydide 
are absent from the Ethiopian region, the Testudinide, a strictly terrestrial family, and the 
Trionychide, wholly aquatic in their habits, abound in that great region. It might seem that 
where the Triony chidz could go the Emydidz could easily follow: However A the Trionychidze 
are a much older family add might have entered Africa before the Emydide had become 
widely distributed and at a time w hen the connections of the continents were greatly different 
from those now existing. It seems establisht that some of the species of Triony chidz have been 
able to adapt ee es to life in sea-water, and this ability may have allowed them to work 
their way along sea-coasts from one fresh-water region to another. 
The Testudinidz, on the other hand, are less dependent on abundance of water for exist- 
ence than are the Emydidz and might have made their way across the Sahara or the deserts of 
western Asia into the Ethiopian region. To the writer it appears more probable that the land- 
tortoises reacht Africa from India by a land connection, of which the Seychelles, Mauritius, the 
Aldabras, and Madagascar are remnants which within recent or Pleistocene times have been 
inhabited by gigantic Testudinide. 
